External cavities have long been used to spectrally narrow and control the wavelength of semiconductor lasers. An external cavity semiconductor laser is comprised of an anti-reflection coated Fabry-Perot semiconductor laser coupled to a wavelength selective external cavity. External cavity laser diodes are capable of single longitudinal mode output over a range of 50 to 100 nm in the communication bands at 1300 and 1550 nm. External cavity lasers have been used in fiber testing and are useful as sources for coherent optical communications. With proper design of the external cavity's wavelength selective elements, the spectral purity of the laser may be quite high. There are several methods that have been used to suppress instabilities in the output of these devices.
The most common frequency selective device to use in an external cavity is a plane diffraction grating that retroreflects light back into the laser diode. The laser is then tuned by rotating the diffraction grating and/or a mirror that is combined with the grating. This type of filter, known as a Littrow grating external cavity, has the advantage of a very narrow bandwidth and simplicity of design. The mode selectivity of a Littrow grating external cavity has been enhanced by placing a solid Fabry-Perot etalon within the cavity. (N. A Olsson et al. "Performance characteristics of 1.5 .mu.m external cavity semiconductor lasers for coherent optical communications", Journal of Lightwave Technology, Vol. LT-5, 510-515, [1987]). The solid etalon is rotated along with the grating to reduce the amplitude of the laser's adjacent longitudinal modes to 40 dB below the power in the main resonance. An alternative geometry for a grating tuned laser is in grazing incidence and is known as the Littman-Metcalf external cavity laser. The advantage here is that the area illuminated by the grating is much larger than in Littrow and that the laser beam double passes the grating. This results in a much higher resolution so that a solid etalon is not needed to suppress the adjacent longitudinal modes. The disadvantage of grating tuned external cavities lies in the inability to tune rapidly since it is limited to frequencies below the bulk mechanical resonances of the system. One also needs to tune the laser with a mechanical actuator, the most common of these being a piezoelectric crystal. The high voltages needed to operate the capacitive piezoelectric crystals require a significant amount of power to operate at high frequencies. Because of the relatively large size and large power consumption, mechanically tuned systems are not ideal for use in hand held battery operated devices. Alternative methods of tuning include electro-optically tuned external cavity lasers.
Tuning of semiconductor lasers using intracavity birefringent filters has been demonstrated using conventional electro-optic materials such as potassium dihydrogen phosphate (KDP). However, these materials require several kilovolts to scan wavelengths over ranges of only 10 nm. The low tuning range and high voltages needed to achieve this range do not result in a compact source. More recently, nematic liquid-crystal based filters have been used to tune external cavity lasers. Liquid crystal based filters require only 10's of volts to tune over a large range. They are capacitive devices so their power consumption is only in the pW range. Since the power consumption is so low, they may be powered using conventional batteries.
An electro-optically tuned laser was developed using a two stage birefringent filter (J. R. Andrews, "Low Voltage Wavelength Tuning of an External Cavity Diode Laser Using a Nematic Liquid Crystal-Containing Birefringent Filter", IEEE Photonic Technology Letters, Vol.2, 334-336, [1990]). Each stage of the filter is composed of a KDP crystal, a liquid crystal, and a polarizer. The two stages are necessary to increase the scan range of the device, but it still exhibits a short scan range of only 2.6 nm. The device also required the use of two KDP crystals, two liquid crystals, and two polarizers. The large number of intracavity elements increases the loss of the cavity due to scattering of light. It also adds cost to the device and makes it complex to construct.
A simpler wavelength selective filter is a liquid crystal Fabry-Perot Interferometer (LC-FPI). The concept of the LC-FPI was first reported by Gunnin et al. in 1981 for the 3-5 .mu.m wavelength range, but performance was poor. Mallison proposed that the LC-FPI could be used as an optical filter for wavelength division multiplexing(WDM), but the bandwidth was too broad for the assigned channel separations. More recent work conducted by Patel et al. (U.S. Pat. No. 5,150,236) and Hirabayashi et al. has improved the performance of these devices so that they are useful in wavelength division multiplexing communication systems.
Reference is made to FIG. 1, depicting the prior art, which shows a schematic of a liquid crystal Fabry Perot interferometer (LC-FPI). A thin liquid crystal layer 1 of refractive index n.sub.lc is inserted between two glass substrates 2. Each of the glass substrates has a conductive layer formed by indium tin oxide (ITO) 3 and a high reflectivity dielectric mirror layer 4 deposited over the ITO layer located adjacent to the liquid crystal. An alignment layer 5 deposited on the dielectric mirror orients the liquid crystal molecules. The ITO layer absorbs at 1550 nm, so it must be placed external to the cavity mirrors. The substrates are aligned so that the reflective surfaces are parallel and separated by distance L.sub.lc. The substrates are usually slighly prismatic to avoid multiple reflections between their surfaces, and the outside surfaces of the substrates have antireflection coatings. The wavelengths .lambda..sub.m of light beam that are transmitted by the LC-FPI form a comb of peaks given by the relationship: EQU .lambda..sub.m =2n.sub.lc L.sub.lc cos(.theta.)/m
where m is an integer and .theta. is the angle if incidence relative to the normal of the reflective surface. Liquid crystals have a large refractive index anisotropy. When no voltage V is applied the liquid crystal molecules orient themselves with the their ordinary axis parallel to the glass substrates. When a voltage V is applied to the liquid crystal, the molecules rotate so that the refractive index gradually changes from the ordinary refractive index to the extraordinary refractive index. The change in the refractive index changes the optical path length between the two mirrors, thereby tuning transmission spectra of the LC-FPI according to the above relationship. Liquid crystal based filters require only 10's of volts to tune over a large range. Since the power consumption is so low, they may be powered using conventional batteries.
The first demonstration of an intracavity LC-FPI to tune a laser was in a fiber ring laser (M. W. Maeda et al., An Electronically Tunable Fiber Laser with a Liquid Crystal Etalon Filter as the Wavelength Selective Element; IEEE Photonics Technology Letters, Vol. 2, 787-789 [1990]). This laser exhibited a wide tuning range and low power consumption, but it was susceptible to multimode behavior because of the long optical path length of the external cavity, L.sub.op. The long cavity length results in the free spectral range of the external cavity being smaller than the resolution bandwidth of the LC-FPI. A more compact cavity design using a retroreflecting end mirror and an intracavity LC-FPI. (Tsuda et al., "Tunable Light Source Using a Liquid-Crystal Fabry-Perot Interferometer", IEEE Photonics Technology Letters, Vol. 3, 504-506, [1991]). In this design, the transmission of an intracavity LC-FPI selects a wavelength that may be retroreflected by the cavity's end mirror. The gain medium of the laser was a multi-quantum well (MQW) laser diode. The laser device still exhibited multimode behavior because the external cavity free spectral range is still much less than the resolution bandwidth of the LC-FPI. To compensate for the multimode behavior, a separate phase control (PC) section on the laser was needed for shifting the internal modes of the laser diode. Single mode operation is obtained by tuning the transmission peak of the LC-FPI and then adjusting the internal gain ripple of the laser diode by applying voltage to the PC section. The problem with such an approach is that it necessitates a method to constantly monitor the mode structure of the output beam and compensate the PC section accordingly. This limits the device's compactness and usability in the field.
Representative of the art is U.S. Pat. No. 5,150,236 to Patel (1992) which describes a tunable liquid-crystal etalon filter comprising two dielectric stack mirrors defining an optical cavity into which is filled a liquid crystal. Application of an electric field to the crystal changes its refractive index and, hence, its optical length.
Also representative of the art is:
U.S. Pat. No. 5,321,539 to Hirabayashi et al (1994) discloses a tunable wavelength filter having a liquid crystal layer.
U.S. Pat. No. 5,068,749 to Patel (1991) discloses an electronically tunable polarization-insensitive optical filter.
U.S. Pat. No. 4,779,959 to Saunders (1988) discloses an electro-optic modulator comprising a Fabry-Perot etalon.
U.S. Pat. No. 4,550,410 to Chenausky et al (1985) discloses a laser using a Fabry-Perot etalon coupled to an optical cavity.